Method of coating thermoplastic webs



Jan. 25, 1966 GRENLEY ETAL 3,231,415

METHOD OF COATING THERMOPLASTIC WEBS Filed Aug. 15. 1961 51 Z Zlfi B 2 IN VEN TORS. Dav/b5 6i Gran/e9 Dale H. Johnson Wa//ace J Bragg HGIENT United States Patent METHOD OF COATING THERMOPLASTIC WEBS Dallas G. Grenley and Dale H. Johnson, Midland, and

Wallace J. Bragg, Sanford, Mich., assignors to The Dow Chemical Company, Midland, Mich, a corporation of Delaware Filed Aug. 15, 1961, Ser. No. 131,545 4 Claims. (Cl. 117-102) This invention relates to a method of coating thermoplastic webs on both sides. It more particularly relates to a method of simultaneously coating thermoplastic webs on each side employing generally opposed air doctors.

Much difiiculty has been encountered in placing a liquid coating in a uniform manner simultaneously on opposite sides of a thin web. This is particularly true when such webs are thermoplastic film having a thickness of about 3 mils or less. Usually when such webs are coated with an aqueous dispersion of a polymeric composition employing opposed air doctors to remove the excess coating material and smooth the remaining coating, vibration of the web will occur resulting in an uneven coating and frequently the transfer of the coating material to the air doctor opening. Transfer of the coating material to "the air doctor opening generally results in constriction of the air doctor opening and subsequent disruption of the delivered planiform air blast resulting in uneven coating of the web. By way of illustration, a 1 mil polystyrene web, when coated with an aqueous dispersion of a styrene butadiene copolymer to give a 0.1 mil thick dry coating, if done by mechanical means, an uneven coating is generally deposited even when one surface of the web being coated is supported by a backing roll or similar means to maintain even spacing from the air doctor or other coating thickness controlling device. Such a technique requires that coating of each surface be two separate operations. Two separate operations require increased processing time, capital expenditure, and cause at least one face of the coated web to contact with a backing or support means.

It is an object of this invention to rovide a means for simultaneously coating opposite faces of a thin thermoplastic web with a uniform coating of a liquid composition.

Another object of this invention is to provide a method of applying a uniform coating of a liquid composition to both faces of a thermoplastic web and dry said coating prior to contacting either surface of said web with support means.

It is a further object of this invention to provide a configuration for the simultaneous coating of both faces of a thin thermoplastic web employing air doctors which eliminate oscillation of the web and produces a coating of uniform thickness.

These benefits and other advantages are achieved in accordance with the invention by continuously immersing a thin thermoplastic web in a liquid coating composition, forwarding said web in an upward direction, passing said coated web between a pair of generally opposed planiform air blasts directed toward the coated surfaces of the web, said planiform air blasts being downwardly directed at an angle of from about 45 to 86 degrees from the plane of the web, said air blasts having adjacent, non-overlapping areas of impingement on opposite sides of said web, subsequently passing said coated web through a heated zone of sufi'icient intensity to dry and fuse said coating, subsequently cooling said coated web. Further benefits and other advantages of the invention will become more apparent when taken in connection with the accompanying specification and drawing wherein:

The figure shows a schematic representation of the method of the invention. A supply roll provides a thin thermoplastic web 12 which passes into a tank 14 containing a coating composition -15. Partially immersed in the coating composition 15 contained with the tank 14 is a roll 17, the web 12 passes underneath the roll 17 and between a pair of opposed air doctors 18 and 19 which are oriented to supply a pair of opposed planiform air blasts oriented in a generally downward direction. A portion of coating composition 15 designated as 15a is carried up to the air doctors 18 and 19. A coated web 12a is carried upwardly between a pair of opposed heat sources 21 and 22. A dried coated film designated as 12b which leaves the heating zone 23 disposed between the heat sources 21 and 22 passes over a draw roll 30 and is subsequently wound onto the mill roll 31.

In operation of the invention the positioning of the air doctors relative to the web is critical if flap and uneven coating are to be eliminated. The air doctors should be directed toward the film in a generally downward direction and disposed from the plane. of the film at an angle of about from 45 to 86 degrees. Further, the air knives must not be directly opposed 'but are positioned in such a way that the direction planiform air blasts issuing from' the air doctors do not form an overlapping pattern at the' location at which they converge on the coated film. Thus, it is necessary that one air blast be directed to a line which lies above or below the area of impingement of the opcoating bath, in order that the excess coating removed by the planiform air blasts may run back into the supply tank from which the web is emerging. The precise distance above the supply bath or tank of coating material at which the air doctors are positioned is generally a compromise between the disruption of the surface of the bath caused by the planiform air blasts and the fact that most oscillation of the web, however it may be induced, usually will be at a maximum at a location midway between the web supports which are the rolls 17 and 30 in the accompanying figure. Advantageously, by supplying gas to the air doctors which is saturated with the vapors of the volatile liquid of the coating composition undesired evaporative effects may be overcome, thus permitting the air doctors to be placed close to the *bath without promoting skinning. Usually with most air doctors employed in the practice of the invention a pressure of about 10 pounds per square inch gage is the maximum pressure generally employed. Pressures above 10 pounds per square inch in the coating of plastic webs in the order of one mil in thickness often exert excessive physical force and accomplish no useful purpose.

The invention is particularly adapted to be practiced with coating compositions having a viscosity of between about 1 centipoise and 2,000 centipoises. Coatings above about 2,000 centipoises are excessively viscous and require relatively high air pressure applied to the air doctors and only relatively slow web coating speeds can be achieved. In viscosity range below about 2,000 centipoises, relatively high coating rates are achieved, such as for example, about 250 to 500 feet per minute and even higher, while maintaining a uniform coating thickness.

The web thickness which may 'be handled in accordance with the invention varies from about 0.2 of a mil to about 10 mils employing conventional thin web handling equipment. Thicker and thinner material webs may be processed if suitable equipment adjustments are made.

Generally, for a web thickness of from 1 to 10 mils a span or distance between the roll 17 and the roll 30 are usually about 10 to 15 feet, but may vary depending on the production desired from about 21 feet to about 100 feet.

In a typical operation in accordance with the invention, a 12 inch wide web of one mil polystyrene film was passed through a bath containing an aqueous colloidal dispersion of styrene-'butadiene copolymer at a rate of about 250 feet per minute. A pair of opposed air doctors were placed about 10 inches above the point where the Web emerges from the bath. The air doctors were adjusted in such a manner that the blast from the lower air doctors was directed toward the web at an angle of about 75 degrees from the plane of the web. The planiform air blast from the upper air doctor impinged on the film above the lower, air blast on the opposite side at an angle of about '84 degrees from the plane of the Web on the opposite side thereof. The coated web was then led upwardly between a pair of radiant heat panels spaced on either side of the web at a distance of about 3 inches. Power input to the radiant heating panels was about 5 kilowatts. The web was then passed over an idler roll and to a rewind stand. The distance between the roll immersed in the coating bath and the idler roll was about 7 feet. Employing a pressure of about 18 inches of water in the air knives and operating a web speed of about 250 feet per minute, a uniform coating having a mil dry thickness was obtained on each side of the polystyrene web. Random samples of the coated web were taken having a dimension of about 3 inches by 3 inches and the coating thickness was measured. The extreme variation of coating thickness was found to be les than 1 percent of the average coating thickness. In a similar manner, webs comprised of other flat materials are readily coated simultaneously on both sides with liquid coating compositions with commensurate results.

As is apparent from the foregoing specification, the method of the present invention is susceptible of being embodied with various alterations and modifications which may differ particularly from those that have been described in the preceding specification and description. For this reason, it is to be fully understood that all of the foregoing is intended to be merely illustrative and is not to be construed or interpreted as being restrictive or 4 otherwise limiting of the present invention, excepting as it is set forth and defined in the hereto appended claims.

What is claimed is:

1. A method of uniformly coating both sides of a thin web comprising: continuously immersing a thin thermoplastic web in a liquid coating composition forwarding said web in an upward direction and in a planar configuration, passing said coated web between a pair of generally opposed unobstructed planiform air blasts directed toward coated surfaces of the web, said planiform air blasts being downwardly directed at an angle of from about 45 to degrees from the plane of the web, said air blasts having adjacent non-overlapping areas of impingement on opposite sides of said web, subsequently passing said coated web through heated zone of sufficient intensity to dry and fuse said coating, maintaining the web in a substantially planar configuration after coating until drying and fusing is completed and subsequently cooling said coated Web.

2. The method of claim 1, wherein said coating composition comprises an aqueous dispersion of a synthetic organic resinous polymer.

3. The method of claim 1, wherein said web is a polystyrene film.

4. The method of claim 3, wherein the dry fused coating has a maximum thickness variation of about 1 percent of the total coating thickness.

References Cited by the Examiner UNITED STATES PATENTS 1,899,449 2/ 1933 Wallsten 118-63 1,936,651 11/1933 Willis 11863 2,097,601 11/1937 Potdevin 11863 2,139,628 12/1938 Terry 118-63 XR 2,176,093 10/1939 Merrill 11863 2,229,921 1/1941 Goff 118--63 2,423,768 7/1947 Goff 118-63 2,541,167 2/1951 Pitzl 117-145 XR 2,628,923 2/1953 Yaeger. 2,829,068 4/1958 Steward. 2,979,410 4/1961 Parlour 117138.8 XR 3,030,325 4/ 1962 Scoop.

WILLIAM D. MARTIN, Primary Examiner.

RICHARD D. NEVIUS, Examiner. 

1. A METHOD OF UNIFORMLY COATING BOTH SIDES OF A THIN WEB COMPRISING: CONTINUOUSLY IMMERSING A THIN THERMOPLASTIC WEB IN A LIQUID COATING COMPOSITION FORWARDING SAID WEB IN AN UPWARD DIRECTION AND IN A PLANAR CONFIGURATION, PASSING SAID COATED WEB BETWEEN A PAIR OF GENERALLY OPPOSED UNOBSTRUCTED PLANIFORM AIR BLASTS DIRECTED TOWARD COATED SURFACES OF THE WEB, SAID PLANIFORM AIR BLASTS BEING DOWNWARDLY DIRECTED AT AN ANGLE OF FROM ABOUT 45 TO 85 DEGREES FROM THE PLANE OF THE WEB, SAID AIR BLASTS HAVING ADJACENT NON-OVERLAPPING AREAS OF IMPINGEMENT ON OPPOSITE SIDES OF SAID WEB, SUBSEQUENTLY PASSING SAID COATED WEB THROUGH HEATED ZONE OF SUFFICIENT INTENSITY TO DRY AND FUSE SAID COATING, MAINTAINING THE WEB IN A SUB- 